I love to use WINDOWS XP so again done this but this time have hard disk some big
Then once you're done with getting the blockchain, shut QT down, replace the empty wallet it created in your
C:\Documents and Settings\YourUserName\Application data\Bitcoin
folder by the one from your usb stick.
I would also make another copy the wallet.dat somewhere else on your harddrive (I don't really trust usb sticks with such "important" data)
After that, create/edit the shortcut you're using to launch QT, and add " -rescan" in the end of the "Target" input box.
Should look like:
...\bitcoin-qt.exe -rescan
where ... is the location of your bitcoin-qt client.
Your balance should be ok, then.
If not, don't panic, QT is sometimes a bit dodgy when it comes to computing balances, we'll find another way.